energies Article Thermodynamic and Economic Analyses of a New Waste-to-Energy System Incorporated with a Biomass-Fired Power Plant Peiyuan Pan 1 , Meiyan Zhang 1 , Gang Xu 1, *, Heng Chen 1, * , Xiaona Song 2 and Tong Liu 1 1 Beijing Key Laboratory of Emission Surveillance and Control for Thermal Power Generation, North China Electric Power University, Beijing 102206, China; [email protected] (P.P.); [email protected] (M.Z.); [email protected] (T.L.) 2 Electrical and Mechanical Practice Center, Beijing Information Science & Technology University, Beijing 100192, China; [email protected]* Correspondence: [email protected] (G.X.); [email protected] (H.C.); Tel.: +86-(10)-6177-2472 (G.X.); +86-(10)-6177-2284 (H.C.) Received: 7 July 2020; Accepted: 20 August 2020; Published: 22 August 2020 Abstract: A novel design has been developed to improve the waste-to-energy process through the integration with a biomass-fired power plant. In the proposed scheme, the superheated steam generated by the waste-to-energy boiler is fed into the low-pressure turbine of the biomass power section for power production. Besides, the feedwater from the biomass power section is utilized to warm the combustion air of the waste-to-energy boiler, and the feedwater of the waste-to-energy boiler is offered by the biomass power section. Based on a 35-MW biomass-fired power plant and a 500-t/d waste-to-energy plant, the integrated design was thermodynamically and economically assessed. The results indicate that the net power generated from waste can be enhanced by 0.66 MW due to the proposed solution, and the waste-to-electricity efficiency increases from 20.49% to 22.12%. Moreover, the net present value of the waste-to-energy section is raised by 5.02 million USD, and the dynamic payback period is cut down by 2.81 years. Energy and exergy analyses were conducted to reveal the inherent mechanism of performance enhancement. Besides, a sensitivity investigation was undertaken to examine the performance of the new design under various conditions. The insights gained from this study may be of assistance to the advancement of waste-to-energy technology. Keywords: waste-to-energy; biomass-fired power generation; steam cycle integration; performance improvement; performance evaluation 1. Introduction Under the stress of energy shortage, global warming, and environmental pollution, the utilization of renewable energy sources has drawn much attention worldwide [1,2]. Biomass is renewable and can be extensively obtained in numerous forms and types [3]. Biomass resources include forestry, agricultural crops and residues, animal residues, industrial waste and residues, municipal solid waste (MSW), sewage, etc. [4]. Typically, biomass can be processed in several approaches, such as combustion, gasification, pyrolysis, liquefaction, torrefaction, steam explosion, hydrothermal carbonization, and anaerobic digestion [5]. Final products derived from biomass involve heat, power, and/or fuel for further usage [6]. Currently, among all thermochemical methods, direct combustion is the most broadly adopted for biomass conversion, which accounts for above 97% of the biomass utilization as energy production globally [4]. Biomass direct combustion is a dominating bioenergy technology for power generation, and it has great compatibility with traditional thermal power production [7]. It is predicted that biomass Energies 2020, 13, 4345; doi:10.3390/en13174345 www.mdpi.com/journal/energies
Transcript
energies
Article
Thermodynamic and Economic Analyses of a NewWaste-to-Energy System Incorporated with aBiomass-Fired Power Plant
Peiyuan Pan 1, Meiyan Zhang 1, Gang Xu 1,*, Heng Chen 1,* , Xiaona Song 2 and Tong Liu 1
1 Beijing Key Laboratory of Emission Surveillance and Control for Thermal Power Generation,North China Electric Power University, Beijing 102206, China; [email protected] (P.P.);[email protected] (M.Z.); [email protected] (T.L.)
2 Electrical and Mechanical Practice Center, Beijing Information Science & Technology University,Beijing 100192, China; [email protected]
Received: 7 July 2020; Accepted: 20 August 2020; Published: 22 August 2020�����������������
Abstract: A novel design has been developed to improve the waste-to-energy process through theintegration with a biomass-fired power plant. In the proposed scheme, the superheated steamgenerated by the waste-to-energy boiler is fed into the low-pressure turbine of the biomass powersection for power production. Besides, the feedwater from the biomass power section is utilized towarm the combustion air of the waste-to-energy boiler, and the feedwater of the waste-to-energyboiler is offered by the biomass power section. Based on a 35-MW biomass-fired power plant anda 500-t/d waste-to-energy plant, the integrated design was thermodynamically and economicallyassessed. The results indicate that the net power generated from waste can be enhanced by 0.66 MWdue to the proposed solution, and the waste-to-electricity efficiency increases from 20.49% to 22.12%.Moreover, the net present value of the waste-to-energy section is raised by 5.02 million USD, and thedynamic payback period is cut down by 2.81 years. Energy and exergy analyses were conducted toreveal the inherent mechanism of performance enhancement. Besides, a sensitivity investigation wasundertaken to examine the performance of the new design under various conditions. The insightsgained from this study may be of assistance to the advancement of waste-to-energy technology.
Under the stress of energy shortage, global warming, and environmental pollution, the utilizationof renewable energy sources has drawn much attention worldwide [1,2]. Biomass is renewableand can be extensively obtained in numerous forms and types [3]. Biomass resources includeforestry, agricultural crops and residues, animal residues, industrial waste and residues, municipalsolid waste (MSW), sewage, etc. [4]. Typically, biomass can be processed in several approaches,such as combustion, gasification, pyrolysis, liquefaction, torrefaction, steam explosion, hydrothermalcarbonization, and anaerobic digestion [5]. Final products derived from biomass involve heat, power,and/or fuel for further usage [6]. Currently, among all thermochemical methods, direct combustionis the most broadly adopted for biomass conversion, which accounts for above 97% of the biomassutilization as energy production globally [4].
Biomass direct combustion is a dominating bioenergy technology for power generation, and ithas great compatibility with traditional thermal power production [7]. It is predicted that biomass
power will exceed 15% of the overall electricity supply in industrial countries by the end of 2020 [8].Biomass power generation has a carbon neutralization effect, and regular biomass contains less sulfurand nitrogen than fossil fuels; thereby, biomass power generation can conduce to the reduction ofemissions [9]. Wood and straw are relatively attractive feedstocks for biomass power generation,and agricultural waste products (rice husks, wheat bran, peanut shells, etc.) are usually burned aswell [10]. On the other hand, MSW as a biomass source is characterized by higher moisture and volatilematter contents, higher O/C ratio, lower calorific value, poorer grindability, and greater concentrationsof alkali or toxic metals in the ashes, as compared to wood/straw, which increases the difficultiesin transportation and storage, handling and feeding of materials, flame stability, energy conversion,and plant availability [11]. Hence, the cofiring of MSW and high-quality biomass is hard and probablynot beneficial.
At present, waste-to-energy (WtE) incineration is a popular method for waste management,due to its features of dramatical MSW volume reduction and affordable costs for heat/electricitygeneration, especially for developing countries [12]. Incineration will handle more than half of theMSW produced in China by the end of 2020, based on the Chinese 13th Five-Year Plan (2016–2020) [13].Nevertheless, the waste-to-electricity efficiencies of conventional WtE incineration plants are relativelylow, ranging from 14% to 28% [14], and the poor performance of a WtE plant is mainly caused bythe limited steam parameters, small capacity, and simple steam cycle [15]. Much research has beendevoted to improving the WtE process, chiefly including promoting steam parameters, decreasingexhaust gas heat loss, and a combination with other thermal systems. MSW contains high contentsof Cl, S, Na, K, and other alkali metals, which can form HCl, sulfate, alkali metal salts, and otherpollutants during the incineration, probably inducing severe corrosion and fouling in the boiler [16].Thus, the live steam parameters of a WtE boiler are seriously restricted and generally around 4 MPaand 400 ◦C. Xu et al. [17] proposed a solution that uses phase-change materials-based refractory brickson the water wall of a WtE boiler to raise the steam temperature. Bogalea et al. [18] described a designthat divides the flue gas into two fractions, and the lower corrosive part is delivered to a separatesuperheater to enhance the steam parameters. Another approach to promote the steam temperature isemploying a radiant superheater in the WtE boiler [19]. Besides, the reduction of the exhaust gas heatloss can be achieved through flue gas recirculation [20]; diminishing excess air [21]; and waste heatrecovery, such as heating combustion air/feedwater [22], producing district heat [23], and driving anorganic Rankine cycle [24]. Concerning system integration, Consonni et al. [25] and Poma et al. [26]discussed a hybrid WtE-gas turbine combined cycle scheme, where the steam yielded from the WtEboiler is further heated by the gas turbine exhaust gas and then enters into the steam turbine for powerproduction. Mendecka et al. [27] presented a novel WtE design incorporated with a concentrating solarpower system, in which the solar energy is harvested to superheat the live steam of the WtE boiler.Chen et al. [28] explored the integration of a WtE plant and a coal-fired power plant by feeding the heatacquired from the incineration into the steam cycle of the coal power section. However, little work hasbeen published on integrating a WtE plant with a biomass-fired power plant based on the steam cycleincorporation, which may contribute to improving the waste-to-electricity efficiency.
This paper proposed an innovative WtE design, which is combined with a biomass-fired powerplant based on the steam cycle. In the integrated scheme, the superheated steam exported from theWtE section enters into the turbine of the biomass power section for producing electricity. Meanwhile,the combustion air of the WtE boiler absorbs energy from the feedwater extracted from the biomasspower section and the saturated steam taken from the WtE boiler, and the biomass power sectionsupplies feedwater for the WtE boiler. Consequently, the waste-to-electricity efficiency may gainsignificant improvement, and some equipment of the WtE plant can be saved, resulting in morerevenues and lower costs. To check the feasibility of the novel concept, thermodynamic and economicevaluations were undertaken based on a 500-t/d WtE plant and a 35-MW biomass-fired power plant.Energy and exergy analyses were performed to investigate the root cause of efficiency enhancement.Additionally, the influence of the boiler loads on the performance of the new design was examined.
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2. Reference Plants and Concept Proposal
2.1. Reference WtE Plant
A typical WtE plant with a daily disposal capacity of 500 t has been selected for the case study,which is sketched in Figure 1. The feedwater enters into the WtE boiler as a working fluid and isheated by the flue gas generated from the waste incineration. After absorbing the heat energy in theboiler, the yielded steam flows into the turbine for power production. The exhaust gas of the WtEboiler is discharged after the treatment of the flue gas scrubber (FGS) and the bag filter (BF). Table 1provides the basic data of the WtE plant. The parameters of the superheated steam at the turbine inletare merely 3.90 MPa and 395.0 ◦C, and the waste-to-electricity efficiency of the plant can only get20.49%. Besides, the feedwater into the WtE boiler is preheated by two regenerative heaters (RHs),the deaerator (DEA) and RH2, and their parameters are given in Table 2.
Energies 2020, 13, x FOR PEER REVIEW 3 of 20
Energy and exergy analyses were performed to investigate the root cause of efficiency enhancement. Additionally, the influence of the boiler loads on the performance of the new design was examined.
2. Reference Plants and Concept Proposal
2.1. Reference WtE Plant
A typical WtE plant with a daily disposal capacity of 500 t has been selected for the case study, which is sketched in Figure 1. The feedwater enters into the WtE boiler as a working fluid and is heated by the flue gas generated from the waste incineration. After absorbing the heat energy in the boiler, the yielded steam flows into the turbine for power production. The exhaust gas of the WtE boiler is discharged after the treatment of the flue gas scrubber (FGS) and the bag filter (BF). Table 1 provides the basic data of the WtE plant. The parameters of the superheated steam at the turbine inlet are merely 3.90 MPa and 395.0 °C, and the waste-to-electricity efficiency of the plant can only get 20.49%. Besides, the feedwater into the WtE boiler is preheated by two regenerative heaters (RHs), the deaerator (DEA) and RH2, and their parameters are given in Table 2.
FGS
Drum
BF
WtE boiler
SAH
PAH3
Turbine
RH2
Generator
DEA(RH1)
CP
Condenser
FWP
Stack
BF: bag filter DEA: deaerator CP: condensate pump FGS: flue gas scrubber FWP: feedwater pump PAH: primary air heater RH: regenerative heater SAH: secondary air heater WtE: waste-to-energy
PAH2
PAH1
Figure 1. Reference WtE plant.
Table 1. Basic parameters of the reference waste-to-energy (WtE) plant. MSW: municipal solid waste.
Item Unit Value Feedstock (MSW) flow rate t/h 20.84
Lower heating value MJ/kg 7.00
Fuel energy input MW 40.51
Live steam$$ (into turbine)
Flow rate t/h 48.60
Pressure MPa 3.90
Temperature °C 395.0
Exhaust steam$$ (into condenser)
Flow rate t/h 35.75
Pressure kPa 6.80
Temperature °C 38.5
Exhaust gas temperature (out of boiler) °C 190.0
Boiler capacity (heat absorbed by working fluid) MW 37.47
Figure 1. Reference WtE plant.
Table 1. Basic parameters of the reference waste-to-energy (WtE) plant. MSW: municipal solid waste.
Item Unit Value
Feedstock (MSW) flow rate t/h 20.84
Lower heating value MJ/kg 7.00
Fuel energy input MW 40.51
Live steam(into turbine)
Flow rate t/h 48.60
Pressure MPa 3.90
Temperature ◦C 395.0
Exhaust steam(into condenser)
Flow rate t/h 35.75
Pressure kPa 6.80
Temperature ◦C 38.5
Exhaust gas temperature (out of boiler) ◦C 190.0
Boiler capacity (heat absorbed by working fluid) MW 37.47
Boiler efficiency % 78.53
Net power output MW 8.30
Waste-to-electricity efficiency % 20.49
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Table 2. Parameters of the heat regeneration system in the reference WtE plant. DEA: deaerator andRH: regenerative heater.
Item DEA (RH1) RH2
Extraction steam
Flow rate (t/h) 3.49 3.74
Pressure (MPa) 0.27 0.08
Temperature (◦C) 195.4 92.8
Drain waterFlow rate (t/h) - 3.74
Temperature (◦C) - 92.8
FeedwaterOutlet flow rate (t/h) 51.1 39.74
Outlet temperature (◦C) 130.0 88.0
The essential combustion air is necessary to be preheated for the stable combustion in the boilerfurnace. Table 3 shows the data of the air preheating system in the reference WtE plant. The primary airis heated by the steam extraction of the turbine and the steam from the drum in the three-stage primaryair heater (PAH), and its temperature is eventually raised to 220.0 ◦C. The secondary air is warmedfrom 15.0 ◦C to 166.0 ◦C by the steam extraction from the turbine in the secondary air heater (SAH).
Table 3. Parameters of the air preheating system in the reference WtE plant. PAH: primary air heaterand SAH: secondary air heater.
ItemPAH
SAHPAH1 PAH2 PAH3
Hot fluid(steam/water)
Inlet pressure (MPa) 4.54 1.31 4.54 1.31
Inlet flow rate (t/h) 2.23 3.89 2.23 1.76
Inlet/outlet temperature (◦C) 225.3/104.3 287.3/98.3 258.0/225.3 287.3/98.5
Inlet/outlet temperature (◦C) 15.0/31.0 31.0/166.0 166.0/220.0 15.0/166.0
2.2. Reference Biomass-Fired Power Plant
Figure 2 displays the diagram of the reference biomass-fired power plant, which mainly involvesa vibrating grate boiler, an extraction turbine, and a generator. The feedstock into the biomass powerplant is primarily stalk. Table 4 presents the fundamental data of the biomass power plant, and theparameters of the live steam into the turbine can reach 535.0 ◦C and 9.40 MPa. In the heat regenerationsystem, the feedwater out of the condensate pump (CP) passes through RH6-1 successively to get itsdesign temperature, and the data of the heat regeneration system is collected in Table 5. The biomasspower plant runs much more efficiently than the WtE plant, and its thermal efficiency can attain 30.13%.
2.3. Proposed Hybrid System
A novel WtE design has been put forward to improve the energy utilization of the MSW, as depictedin Figure 3. Several connections between the WtE section and the biomass power section have beenestablished, aiming at the steam cycle. In the new scheme, the steam turbine of the biomass powerplant is divided into two parts, the high-pressure turbine (HPT) and the low-pressure turbine (LPT).The superheated steam derived from the WtE boiler is sent to the LPT for power production, and thebiomass power section provides feedwater for the WtE boiler. Moreover, a three-stage PAH and adouble-stage SAH are arranged to accomplish the air preheating for the WtE boiler. Partial feedwaterobtained from the RH6 outlet is taken to the PAH1 and SAH1 to warm the air; afterward, the feedwaterextracted from the DEA outlet is transferred to the PAH2 and SAH2 to raise the air temperature.The primary air is further heated to 220.0 ◦C in the PAH3 by the saturated steam of the WtE boiler,
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and then, the condensate out of the PAH3 is delivered to the DEA inlet after decreasing its pressure bythe pressure reducing valve (PRV). The condensate out of the PAH1 and SAH1 is pumped into theRH6 by the additional pump (AP), and the condensate of the PAH2 and SAH2 is supplied to the WtEboiler as feedwater. The exhaust gas of the WtE boiler is still treated by the FGS and BF and is finallyexpelled via the stack of the biomass power plant. Attributed to the proposed solution, the WtE sectionand biomass power section exploit the same steam turbine, and the overall power production couldbe improved. Besides, some components (turbine, RHs, condenser, generator, stack, etc.) of the WtEplant are not needed after the hybridization. Above all, the suggested integration may contribute to aremarkable increment in the waste-to-electricity efficiency, and the capital costs of the WtE systemcould be dramatically cut down.Energies 2020, 13, x FOR PEER REVIEW 5 of 20
Generator
DEA(RH3) RH6RH5RH2RH1
BF
Condenser
Biomass-fired boiler
FGDStack
CP
FWP
RH4
DrumTurbine
FGD: flue gas desulfurization
Figure 2. Reference biomass-fired power plant.
Table 4. Basic parameters of the reference biomass-fired power plant.
Item Unit Value Feedstock (biomass) flow rate t/h 39.13
Lower heating value MJ/kg 9.435
Fuel energy input MW 102.55
Live steam$$ (into turbine)
Flow rate t/h 128.99
Pressure MPa 9.40
Temperature °C 535.0
Exhaust steam$$
(into condenser)
Flow rate t/h 91.80
Pressure kPa 4.90
Temperature °C 32.5
Exhaust gas temperature (out of boiler) °C 127.0
Boiler capacity (heat absorbed by working fluid) MW 91.36
Boiler efficiency % 89.10
Net power output MW 30.90
Biomass-to-electricity efficiency % 30.13
Table 5. Parameters of the heat regeneration system in the reference biomass-fired power plant.
Item RH1 RH2 DEA (RH3) RH4 RH5 RH6
Extraction steam
Flow rate (t/h) 7.88 5.83 3.64 5.04 5.80 9.00
Pressure (MPa) 2.55 1.32 0.59 0.40 0.19 0.07
Temperature (°C) 381.4 353.5 278.2 190.5 126.8 88.2
Exhaust gas temperature (out of boiler) (°C) 127.0 127.0 0.00
Figure 3. Proposed WtE system integrated with a biomass-fired power plant.
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3. System Simulation
To assess the performance of the integrated scheme as compared to that of the separatescheme (including the conventional WtE plant and biomass-fired power plant), the softwareEBSILON Professional (Version 13.0; STEAG Energy Services GmbH) was employed to carry out thesimulation/calculation. EBSILON Professional has an uninterrupted model structure with standardcomponents that are used for modeling various thermal systems, and it can balance a model withthe help of a closed solution system (based on a sequential solution method) [29]. In this software,the reference biomass power plant, the reference WtE plant, and the hybrid system were modeled usingthe built-in components. The built models were validated by comparing the calculation results with thedesign data of the reference plants, and some of the comparisons are presented in Tables 6 and 7. As therelative errors are very small, the simulation models seem to be accurate and available.
Table 6. Comparison of the simulation results and design data of the reference WtE plant.
Item Design Calculated Relative Error (%)
Feedstock (MSW) flow rate (t/h) 20.84 20.84 0.00
Live steam(into turbine)
Flow rate (t/h) 48.60 48.60 0.00
Pressure (MPa) 3.90 3.90 0.00
Temperature (◦C) 395.0 395.0 0.00
Exhaust steam(into condenser)
Flow rate (t/h) 35.68 35.75 +0.20
Pressure (MPa) 6.80 6.80 0.00
Temperature (◦C) 38.5 38.5 0.00
Exhaust gas temperature (out of boiler) (◦C) 190.0 190.0 0.00
For comparing the performances of the two schemes, a few crucial hypotheses have beenbrought forward:
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(a) The feedstock flow rates of the biomass power plant and the WtE plant are fixed.(b) The exhaust gas temperatures and efficiencies of the WtE boiler and the biomass-fired boiler
remain unchanged.(c) The biomass-to-electricity efficiency and power output of the biomass power plant are invariable.(d) The temperature and pressure of the dead state are 288.15 K and 101.325 kPa.(e) The surroundings’ effects are neglected.
4.2. Energy Analysis
Through the simulation of EBSILON Professional, the parameters of the proposed hybrid systemwere determined considering the design data of the reference WtE plant and biomass power plant.Numerous groups of parameters were derived for the hybrid system during the calculations, and theoptimal group has been selected and presented in this paper.
In the new design, the feedwater from the heat regeneration system of the biomass power sectionand the saturated steam from the WtE boiler are utilized to warm the air fed into the WtE boiler.Table 8 shows the parameters of the air preheating process in the WtE section of the integrated scheme.In PAH1 and SAH1, the temperatures of the primary air and the secondary air are increased to 74.1 ◦Cby absorbing heat from the feedwater fetched from the RH6 outlet. Then, their temperatures augmentto 166.0 ◦C through the heating in the PAH2 and SAH2. Furthermore, the hot primary air is heated to220.0 ◦C by the saturated steam provided by the WtE boiler.
Table 8. Parameters of the air preheating system in the WtE section of the integrated scheme.
ItemPAH SAH
PAH1 PAH2 PAH3 SAH1 SAH2
Hot fluid(water/steam)
Inlet pressure (MPa) 0.89 0.87 4.54 0.89 0.87
Inlet flow rate (t/h) 18.04 36.29 2.23 7.38 14.83
Inlet/outlet temperature (◦C) 93.2/35.0 174.0/130.1 258.0/225.3 93.2/35.0 174.0/130.1
Inlet/outlet temperature (◦C) 15.0/74.1 74.1/166.0 166.0/220.0 15.0/74.1 74.1/166.0
The waste-to-electricity efficiency (ηen,w) and the total energy efficiency (ηen,tot) have been definedto measure the energy performances of the studied systems.
ηen,w =Pw
(mw/3.6) × qw,net(1)
ηen,tot =Ptot
(mb/3.6) × qb,net + (mw/3.6) × qw,net(2)
where Ptot and Pw are the net total power output and the net power output of MSW, kW, qb,net andqw,net are the lower heating values of the biomass and MSW, kJ/kg, mb and mw are the feedstock flowrates of the biomass-fired boiler and WtE boiler, t/h.
The net power output of the biomass remains invariable after the integration; thereby, the powergenerated by the WtE section in the integrated scheme can be determined as:
Pw = Ptot − Pb (3)
where Pb is the net power output of biomass, kW.The thermal performances of the two schemes are presented in Table 9. As the feedstock (biomass
and MSW) flow rates in the two schemes are maintained constant, the gross total power production ofthe integrated scheme is 0.47 MW larger than that of the separate one. Due to the removal of severaldevices (for instance, the circulating water pump and CP), the total parasitic power consumption
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declines from 4.90 MW to 4.71 MW. Hence, the net total power output is improved from 39.20 MW to39.86 MW, and the total energy efficiency grows from 27.40% to 27.86%. Under the condition that thenet power output of biomass is considered as unchanged after the hybridization, the waste-to-electricityefficiency increases by 1.63 percentage points.
Table 9. Thermal performances of the two schemes.
Item Unit Separate Scheme Integrated Scheme Difference
Several scholars have attempted to improve the WtE process by the integration with other powersystems, such as a gas turbine combined cycle or coal-fired power plant. The performance of the currenthybrid system was compared to the performances of two different hybrid systems in reference [26] andreference [28], as displayed in Table 10. By integrating the WtE system with a gas turbine combinedcycle or coal-fired power plant, the waste-to-electricity efficacy can be promoted by 6.9 percentagepoints or 9.16 percentage points, both of which are larger than the efficiency improvement dueto the incorporation with a biomass power plant in this paper. This is mainly because the steamparameters are relatively high in the combined cycle or coal power plant, and their steam cycles aremore efficient. Whereas, the proposal in this paper provides another solution to enhance the WtEtechnology, especially when building a new WtE plant near an existing biomass-fired power plant.Furthermore, the current approach can achieve a hybrid system completely by using renewable energysources (biomass and MSW), which can play an important role in addressing the issues of energyshortage and global warming.
Table 10. Performance comparison of the WtE systems integrated with various power systems.
Item Ref [26] Ref [28] Current paper
Integrated scheme WtE + gas turbinecombined cycle
WtE + coal-firedpower plant
WtE + biomass-firedpower plant
Fuels MSW + natural gas MSW + coal MSW + biomass
Waste-to-electricity efficiency ofconventional WtE system (%) 26.9 20.49 20.49
Waste-to-electricity efficiency ofintegrated WtE system (%) 33.8 29.65 22.12
Improvement in waste-to-electricityefficiency due to integration (%) 6.9 9.16 1.63
The detailed energy flows in the two schemes are displayed in Figure 4. The feedstocks (biomassand MSW) provide the energy inputs of the two schemes, which are identical for the two schemes.In the separate scheme, the superheated steam of 42.61 MW from the WtE boiler goes into the turbineto produce work, and the extraction steam of 4.08 MW from the turbine is conveyed to warm thecombustion air in the PAH2 and SAH. In the integrated scheme, 42.61-MW energy from the WtE boiler is
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delivered to the turbine of the biomass power section for power generation. Meanwhile, the feedwaterderived from the biomass power section is sent to preheat the air into the WtE boiler, then flowsinto the WtE boiler. Since the total steam flow rate into the condensers declines by 0.83 t/h after theincorporation, the overall energy loss of the condensers decreases from 77.64 MW to 77.16 MW. Besides,the energy losses of the turbines and the boilers remain unchanged in the two schemes. Generally,the integrated scheme generates an additional power of 0.66 MW compared to the separate scheme.
Energies 2020, 13, x FOR PEER REVIEW 10 of 20
Table 10. Performance comparison of the WtE systems integrated with various power systems.
Item Ref [26] Ref [28] Current paper
Integrated scheme WtE + gas turbine combined cycle
WtE + coal-fired power
plant
WtE + biomass-fired power
plant
Fuels MSW + natural
gas MSW + coal MSW + biomass
Waste-to-electricity efficiency of conventional WtE system (%) 26.9 20.49 20.49
Waste-to-electricity efficiency of integrated WtE system (%) 33.8 29.65 22.12
Improvement in waste-to-electricity efficiency due to integration (%) 6.9 9.16 1.63
The detailed energy flows in the two schemes are displayed in Figure 4. The feedstocks (biomass and MSW) provide the energy inputs of the two schemes, which are identical for the two schemes. In the separate scheme, the superheated steam of 42.61 MW from the WtE boiler goes into the turbine to produce work, and the extraction steam of 4.08 MW from the turbine is conveyed to warm the combustion air in the PAH2 and SAH. In the integrated scheme, 42.61-MW energy from the WtE boiler is delivered to the turbine of the biomass power section for power generation. Meanwhile, the feedwater derived from the biomass power section is sent to preheat the air into the WtE boiler, then flows into the WtE boiler. Since the total steam flow rate into the condensers declines by 0.83 t/h after the incorporation, the overall energy loss of the condensers decreases from 77.64 MW to 77.16 MW. Besides, the energy losses of the turbines and the boilers remain unchanged in the two schemes. Generally, the integrated scheme generates an additional power of 0.66 MW compared to the separate scheme.
Generator(MSW)
Waste-to-electricity efficiency：20.49%Total energy efficiency：27.40%
MSW40.51 MW(28.32%)
Biomass102.55 MW
(71.68%)
Loss8.70 MW(6.08%)
Loss12.09 MW
(8.45%)
Loss (Condenser)21.93 MW(15.33%)
Loss (Turbine)0.02 MW(0.01%)
Loss 0.10 MW(0.07%)
Steam 42.61 MW (29.79%)
Steam 4.08 MW (2.85%)
Water 6.72 MW (4.70%)
Work9.86 MW (6.90%)
Auxiliary power 1.46 MW (1.02%)
Net total power
39.20 MW (27.40%)
Net power 8.30 MW (5.80%)
Loss (Condenser)55.71 MW(38.94%)
Loss (Turbine)0.07 MW(0.05%)
Loss0.35 MW(0.24%)
Net power 30.90 MW (21.60%)
Auxiliary power 3.43 MW (2.40%)
Work34.68 MW (24.24%)
Steam 122.10 MW
(85.35%)
Water31.65 MW (22.12%)
Sum: 20.79 MW(14.53%)
Sum: 77.64 MW(54.27%)
Sum: 0.09 MW(0.06%)
Sum: 0.42 MW(0.31%)
Generator(Biomass)
Turbine(Biomass)
Boiler(Biomass)
Boiler(MSW)
Turbine(MSW)
(a) Energies 2020, 13, x FOR PEER REVIEW 11 of 20
Waste-to-electricity efficiency：22.12%Total energy efficiency：27.86%
MSW40.51 MW(28.32%)
Biomass102.55 MW
(71.68%)
Water 11.36 MW (7.94%)
Water0.56 MW (0.39%)
Steam 42.61 MW (29.79%)
Loss8.70 MW(6.08%)
Loss12.09 MW
(8.45%) Steam 122.10 MW
(85.35%)
Water 31.65 MW (22.12%)
Work45.01 MW (31.47%)
Loss (Condenser)77.16 MW(53.94%)
Loss (Turbine)0.09 MW(0.06%)
Loss0.45 MW(0.31%)
Auxiliary power 4.71 MW (3.29%)
Net total power
39.86 MW (27.86%)
Sum: 20.79 MW(14.53%)
Boiler(MSW)
Boiler(Biomass)
Turbines(Biomass) Generator
(Biomass)
(b)
Figure 4. Energy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme.
4.3. Exergy Analysis
Exergy is regarded as the maximum useful work that can be utilized during the energy conversion process [30], and the exergy analysis can provide extra information as compared to the energy analysis [31].
The exergy of the feedstock ( fEX , kW) can be estimated as [32]:
f f,netf(H) (O) (N)( / 3.6) (1.0064 0.1519 0.0616 0.0429 )(C) (C) (C)w w wEX m qw w w
= × × + × + × + ×
(4)
where fm is the feedstock (MSW or biomass) flow rate, t/h; f,n etq is the lower heating value of the
feedstock, kJ/kg; and (H)w , (C)w , (O)w , and (N)w are the contents of hydrogen, carbon, oxygen, and nitrogen in the feedstock.
The waste-to-electricity exergy efficiency ( ex,wη) and the total exergy efficiency ( ex,totη ) are
formulated as:
wex,w
w
PEX
η =
(5)
totex,tot
b w
PEX EX
η =+
(6)
where wEX and bEX are the exergy of the MSW and the exergy of the biomass, kW. The exergy flows of the two schemes were explored, as illustrated in Figure 5, where the exergy
inflows, exergy outflows, and exergy losses of the main components are indicated. In the separate scheme, the MSW exergy (42.79 MW) and the biomass exergy (111.14 MW) enter into the WtE boiler and the biomass-fired boiler, and the boilers offer exergy (steam) to the steam cycles of the two reference plants, contributing a total exergy output (electricity) of 39.20 MW. In the integrated scheme, a portion of the feedwater is delivered from the DEA outlet into the PAH2 and SAH2 to warm the air, and then, the condensate enters into the WtE boiler; thereby, more steam extractions into RH3–6 will be needed for feedwater heating. Moreover, partial feedwater of the biomass power section is
Figure 4. Energy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme.
Energies 2020, 13, 4345 11 of 20
4.3. Exergy Analysis
Exergy is regarded as the maximum useful work that can be utilized during the energy conversionprocess [30], and the exergy analysis can provide extra information as compared to the energyanalysis [31].
The exergy of the feedstock (EXf, kW) can be estimated as [32]:
EXf = (mf/3.6) × qf,net × (1.0064 + 0.1519×w(H)
w(C)+ 0.0616×
w(O)
w(C)+ 0.0429×
w(N)
w(C)) (4)
where mf is the feedstock (MSW or biomass) flow rate, t/h; qf,net is the lower heating value of thefeedstock, kJ/kg; and w(H), w(C), w(O), and w(N) are the contents of hydrogen, carbon, oxygen,and nitrogen in the feedstock.
The waste-to-electricity exergy efficiency (ηex,w) and the total exergy efficiency (ηex,tot) areformulated as:
ηex,w =Pw
EXw(5)
ηex,tot =Ptot
EXb + EXw(6)
where EXw and EXb are the exergy of the MSW and the exergy of the biomass, kW.The exergy flows of the two schemes were explored, as illustrated in Figure 5, where the exergy
inflows, exergy outflows, and exergy losses of the main components are indicated. In the separatescheme, the MSW exergy (42.79 MW) and the biomass exergy (111.14 MW) enter into the WtE boilerand the biomass-fired boiler, and the boilers offer exergy (steam) to the steam cycles of the two referenceplants, contributing a total exergy output (electricity) of 39.20 MW. In the integrated scheme, a portionof the feedwater is delivered from the DEA outlet into the PAH2 and SAH2 to warm the air, and then,the condensate enters into the WtE boiler; thereby, more steam extractions into RH3–6 will be neededfor feedwater heating. Moreover, partial feedwater of the biomass power section is used to preheat theair in the PAH1 and SAH2; thus, the steam extraction into the RH6 will be augmented. Attributed tothe incorporation, the net total exergy output (electricity) is raised to 39.86 MW, which is 0.66 MWlarger than that of the separate scheme. In addition, the total exergy loss in the boilers declines from93.05 MW to 92.45 MW, which is mainly because the temperature differences in the PAHs and SAHsdrop. The total exergy loss of the condensers dwindles by 0.42 MW due to the decrease of the steaminto the condensers. The total exergy loss of the turbines is raised from 11.05 MW to 11.24 MW, and thetotal exergy loss of the RHs increases from 0.43 MW to 0.79 MW. The exergy losses of other componentshave no obvious changes. Therefore, the total exergy loss of the integrated scheme diminishes from114.73 MW to 114.07 MW. Caused by the integration, the waste-to-electricity exergy efficiency ispromoted from 19.40% to 20.94%, and the total exergy efficiency is enhanced from 25.46% to 25.89%.
Energies 2020, 13, 4345 12 of 20
Energies 2020, 13, x FOR PEER REVIEW 12 of 20
used to preheat the air in the PAH1 and SAH2; thus, the steam extraction into the RH6 will be augmented. Attributed to the incorporation, the net total exergy output (electricity) is raised to 39.86 MW, which is 0.66 MW larger than that of the separate scheme. In addition, the total exergy loss in the boilers declines from 93.05 MW to 92.45 MW, which is mainly because the temperature differences in the PAHs and SAHs drop. The total exergy loss of the condensers dwindles by 0.42 MW due to the decrease of the steam into the condensers. The total exergy loss of the turbines is raised from 11.05 MW to 11.24 MW, and the total exergy loss of the RHs increases from 0.43 MW to 0.79 MW. The exergy losses of other components have no obvious changes. Therefore, the total exergy loss of the integrated scheme diminishes from 114.73 MW to 114.07 MW. Caused by the integration, the waste-to-electricity exergy efficiency is promoted from 19.40% to 20.94%, and the total exergy efficiency is enhanced from 25.46% to 25.89%.
Figure 5. Exergy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme.
5. Economic Analysis
An economic analysis, which considers both the cash inflows and cash outflows within the duration of a project, was conducted to offer another view on the feasibility of the novel concept. Under the condition that the expenses and earnings of the biomass power section were deemed as unchanged, the WtE section was solely studied. Table 11 presents the basic economic data for the evaluation. The reference WtE plant was constructed in China with a unit capital cost of 56,500 USD/t, and the electricity sale and waste disposal fee are the main sources of cash inflows.
Table 11. Basic data for the economic analysis.
Item Unit Value Capital cost of reference WtE plant [33] million USD 28.25
Operational cost of reference WtE plant [33] million USD 2.83
Lifetime of WtE plant [34] Construction year 2
Economic year 23
Annual operating time of WtE plant [34] h 7200
Loan ratio [33] - 70%
Loan term [33] year 15
Interest rate [33] - 6.15%
Discount rate [33] - 12%
Figure 5. Exergy flows of the two schemes. (a) Separate scheme. (b) Integrated scheme.
5. Economic Analysis
An economic analysis, which considers both the cash inflows and cash outflows within theduration of a project, was conducted to offer another view on the feasibility of the novel concept.Under the condition that the expenses and earnings of the biomass power section were deemed asunchanged, the WtE section was solely studied. Table 11 presents the basic economic data for theevaluation. The reference WtE plant was constructed in China with a unit capital cost of 56,500 USD/t,and the electricity sale and waste disposal fee are the main sources of cash inflows.
Due to the novel solution, some components in the WtE section and the biomass power sectionare removed, retrofitted, or added. The capital costs of the changed equipment were calculated bythe methods shown in Table 12, and the results are given in Table 13. Several components (turbine,condenser, stack, etc.) of the WtE plant are removed after the integration; thereby, the capital costs of6649.68 thousand USD can be saved. As some extra components (PRV and AP) are installed for thehybrid configuration, an additional capital cost of 0.83 thousand USD is needed. Moreover, the PAHand SAH of the WtE boiler and the turbine and generator of the biomass power section are remoldedfor the incorporation; the relative capital cost will augment by 4407.10 thousand USD. Consequently,the total capital cost of the WtE section diminishes by 2241.74 thousand USD due to the new design.
Energies 2020, 13, 4345 14 of 20
Table 11. Basic data for the economic analysis.
Item Unit Value
Capital cost of reference WtE plant [33] million USD 28.25
Operational cost of reference WtE plant [33] million USD 2.83
Lifetime of WtE plant [34] Construction year 2
Economic year 23
Annual operating time of WtE plant [34] h 7200
Loan ratio [33] - 70%
Loan term [33] year 15
Interest rate [33] - 6.15%
Discount rate [33] - 12%
Disposal fee [33] USD/t 9.65
Feed-in tariff [35] USD/(kW·h) 0.97
Income tax rate ineconomic duration [34]
1st–3rd year - 0%
4th–6th year - 12.5%
7th–23rd year - 25.0%
Table 12. Capital cost estimation methods for the changed equipment.
Note: CCT, CCP, CCG, CCDEA, CCRH, and CCPRV are the capital costs of the turbine, pump, generator, DEA, RH,and pressure reducing valve (PRV), USD; WT,nom is the nominal work output of the turbine, kW; WP,nom is thenominal work consumption of the pump, kW; PG,nom is the nominal power output of the generator, kW; mfw isthe feedwater flow rate, kg/s; A is the heat transfer area, m2; and pin and pout are the inlet pressure and outletpressure, kPa.
Energies 2020, 13, 4345 15 of 20
Table 13. Capital costs of the changed equipment. CP: condensate pump and AP: additional pump.
Component Separate Scheme(Thousand USD)
Integrated Scheme(Thousand USD) Difference
WtE section
Turbine 4111.07 - −4111.07
Generator 370.30 - −370.30
Condenser 118.78 - −118.78
Circulating water pump 146.65 - −146.65
Cooling tower 1504.68 - −1504.68
CP 9.02 - −9.02
RH 19.08 - −19.08
DEA 32.30 - −32.30
Stack 337.81 - −337.81
PAH 178.94 297.13 +118.19
SAH 42.14 119.01 +76.87
Biomass power sectionTurbine 10036.51 13904.86 +3868.34
Generator 1221.93 1565.63 +343.70
PRV - 0.11 +0.11
AP - 0.72 +0.72
Sum 18129.20 15887.46 −2241.74
To examine the economic feasibility, two indicators have been adopted, the dynamic paybackperiod (DPP, year) and the net present value (NPV, USD) [43]:
DPP∑y=1
Cin −Cout
(1 + rdis)y = 0 (7)
NPV =b∑
y=1
Cin −Cout
(1 + rdis)y (8)
where Cin is the cash inflow, USD, Cout is the cash outflow, USD, y is the year number in the plantlifetime, b is the plant lifetime, year, and rdis is the discount rate.
As shown in Table 14, the total capital cost of the WtE section is cut down by 2.24 million USD dueto the proposal; meanwhile, the operational cost decreases from 2.83 million USD to 2.60 million USD.Since the feedstock (MSW) flow rate is invariable, the MSW disposal capacities in the two schemes areboth 150,000 t, and the waste disposal fee remains fixed. Induced by the hybridization, the electricitysupply is improved from 59.76 GW·h to 64.52 GW·h; thereby, the revenue of electricity sales increaseby 0.46 million USD. Hence, the dynamic payback period of the WtE section declines from 8.18 yearsto 5.37 years, and the net present value grows from 5.26 million USD to 10.28 million USD.
Energies 2020, 13, 4345 16 of 20
Table 14. Economic analysis results of the WtE section in the two schemes.
Item Separate Scheme Integrated Scheme Difference
Total capital cost (million USD) 28.25 26.01 −2.24
Operational cost (million USD) 2.83 2.60 −0.23
Annual amount of MSW disposal (103 t) 150 150 0.00
Annual revenue due to MSW disposal (million USD) 1.45 1.45 0.00
Annual revenue due to electricity sale (million USD) 5.77 6.23 +0.46
Total annual revenue (million USD) 7.22 7.68 +0.46
Net present value (million USD) 5.26 10.28 +5.02
Dynamic payback period (year) 8.18 5.37 −2.81
6. Sensitivity Analysis
Regarding the operation of the hybrid system, the dominating factors are the conditions of thebiomass-fired boiler and the WtE boiler. The loads of the two boilers basically impact the live steamparameters; thereby, the performance of the steam cycle is mostly determined. The boiler loads aremainly dependent on the fuel qualities and quantities. Normally, while the fuel qualities/types of thetwo boilers change in limited ranges, the boiler loads can be adjusted by regulating the fuel feed rates.On the condition that the boiler loads are constant, the live steam parameters of the two boilers can benearly maintained identical. Hence, a sensitivity analysis has been undertaken to check how the boilerloads affect the performance of the hybrid system. However, the effects of the fuel types or qualitieswere not discussed. Besides, the fuel types or qualities are necessary to remain relatively stable for thetwo boilers during operation. If the fuel types or qualities are far from the design ones, the two boilerscannot work well or may even shut down, especially the WtE boiler.
Figure 6 displays the influence of the biomass-fired boiler load on the performance of the hybridsystem. It can be seen that the waste-to-electricity efficiency decreases when the biomass-fired boilerload declines from 100% to 90%, but then, the waste-to-electricity efficiency grows along with thedecrement of the load. This is chiefly because the temperature of the feedwater that heats the combustionair of the WtE boiler in the PAH2 and SAH2 diminishes with the descending biomass-fired boilerload, and the feedwater from the RH2 outlet is employed for air preheating once the biomass-firedboiler load ranges from 60% to 90%. As a result, the power consumption of the FWP1 increases,which reduces the net power output of the MSW. Besides, with the decrease of the biomass-firedboiler load condition (from 90% to 60%), the outlet air temperatures of the PAH1/SAH1will be higher;thereby, more low-grade heat from the biomass-fired section can be used to warm the air instead of thehigh-grade heat, which is favorable to producing more power.
The performance of the hybrid system varying with the WtE boiler load is presented in Figure 7.With the decrease of the WtE boiler load, the DEA inlet steam pressure and the feedwater temperatureinto the PAH2/SAH2 dwindle. Furthermore, the air temperature between the PAH1 and PAH2 (or SAH1and SAH2) gets larger, leading to utilizing more low-grade heat from the biomass power section.As a consequence, while the WtE boiler load falls, the net power output of the MSW drops, but thewaste-to-electricity efficiency rises slightly.
Energies 2020, 13, 4345 17 of 20Energies 2020, 13, x FOR PEER REVIEW 17 of 20
60% 70% 80% 90% 100%8.4
8.6
8.8
9.0
9.2
9.4 Net power output of MSW Waste-to-electricity efficiency
Load of biomass-fired boiler
Net
pow
er o
utpu
t of M
SW (M
W)
20.5
21.0
21.5
22.0
22.5
Was
te-to
-ele
ctric
ity e
ffici
ency
(%)
Figure 6. Effect of the biomass-fired boiler load on the performance of the integrated WtE system.
The performance of the hybrid system varying with the WtE boiler load is presented in Figure 7. With the decrease of the WtE boiler load, the DEA inlet steam pressure and the feedwater temperature into the PAH2/SAH2 dwindle. Furthermore, the air temperature between the PAH1 and PAH2 (or SAH1 and SAH2) gets larger, leading to utilizing more low-grade heat from the biomass power section. As a consequence, while the WtE boiler load falls, the net power output of the MSW drops, but the waste-to-electricity efficiency rises slightly.
60% 70% 80% 90% 100%5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
Net power output of MSW Waste-to-electricity efficiency
Load of WtE boiler
Net
pow
er o
utpu
t of M
SW (M
W)
21.6
21.8
22.0
22.2
22.4
22.6
22.8
23.0 W
aste
-to-e
lect
ricity
effi
cien
cy (%
)
Figure 7. Effect of the WtE boiler load on the performance of the integrated WtE system.
7. Conclusions
For improving the energy utilization efficiency of the waste-to-energy process, this paper developed an innovative design that combines a waste-to-energy plant with a biomass-fired power plant based on the steam cycle integration. According to the novel concept, the superheated steam generated by the waste-to-energy boiler enters into the turbine of the biomass power section for power production. Meanwhile, the air fed into the waste-to-energy boiler is warmed by the feedwater from the biomass power section and the saturated steam from the waste-to-energy boiler, and the biomass power section supplies feedwater for the waste-to-energy boiler. To assess the new design
Figure 6. Effect of the biomass-fired boiler load on the performance of the integrated WtE system.
Energies 2020, 13, x FOR PEER REVIEW 17 of 20
60% 70% 80% 90% 100%8.4
8.6
8.8
9.0
9.2
9.4 Net power output of MSW Waste-to-electricity efficiency
Load of biomass-fired boiler
Net
pow
er o
utpu
t of M
SW (M
W)
20.5
21.0
21.5
22.0
22.5
Was
te-to
-ele
ctric
ity e
ffici
ency
(%)
Figure 6. Effect of the biomass-fired boiler load on the performance of the integrated WtE system.
The performance of the hybrid system varying with the WtE boiler load is presented in Figure 7. With the decrease of the WtE boiler load, the DEA inlet steam pressure and the feedwater temperature into the PAH2/SAH2 dwindle. Furthermore, the air temperature between the PAH1 and PAH2 (or SAH1 and SAH2) gets larger, leading to utilizing more low-grade heat from the biomass power section. As a consequence, while the WtE boiler load falls, the net power output of the MSW drops, but the waste-to-electricity efficiency rises slightly.
60% 70% 80% 90% 100%5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
Net power output of MSW Waste-to-electricity efficiency
Load of WtE boiler
Net
pow
er o
utpu
t of M
SW (M
W)
21.6
21.8
22.0
22.2
22.4
22.6
22.8
23.0
Was
te-to
-ele
ctric
ity e
ffici
ency
(%)
Figure 7. Effect of the WtE boiler load on the performance of the integrated WtE system.
7. Conclusions
For improving the energy utilization efficiency of the waste-to-energy process, this paper developed an innovative design that combines a waste-to-energy plant with a biomass-fired power plant based on the steam cycle integration. According to the novel concept, the superheated steam generated by the waste-to-energy boiler enters into the turbine of the biomass power section for power production. Meanwhile, the air fed into the waste-to-energy boiler is warmed by the feedwater from the biomass power section and the saturated steam from the waste-to-energy boiler, and the biomass power section supplies feedwater for the waste-to-energy boiler. To assess the new design
Figure 7. Effect of the WtE boiler load on the performance of the integrated WtE system.
7. Conclusions
For improving the energy utilization efficiency of the waste-to-energy process, this paper developedan innovative design that combines a waste-to-energy plant with a biomass-fired power plant basedon the steam cycle integration. According to the novel concept, the superheated steam generated bythe waste-to-energy boiler enters into the turbine of the biomass power section for power production.Meanwhile, the air fed into the waste-to-energy boiler is warmed by the feedwater from the biomasspower section and the saturated steam from the waste-to-energy boiler, and the biomass power sectionsupplies feedwater for the waste-to-energy boiler. To assess the new design thermodynamically andeconomically, the separate schemes and the integrated scheme were simulated based on a 35-MWbiomass-fired power plant and a 500-t/d waste-to-energy plant. When the feedstock (biomass andmunicipal solid waste) flow rates are constant, the net power output of municipal solid wasteis promoted by 0.66 MW, and the waste-to-electricity efficiency increases from 20.49% to 22.12%.Energy and exergy analyses were implemented to uncover the root cause of efficiency-boosting.The decrease of steam into the condensers is the main cause of energy loss reduction. On the otherhand, the exergy losses of the boilers and the condensers are diminished significantly, induced by the
Energies 2020, 13, 4345 18 of 20
decreased temperature differences of the air preheaters and the decline of the steam into the condensers.The economic analysis results indicate that the dynamic payback period of the waste-to-energy sectiondiminishes from 8.18 years to 5.37 years, and the net present value is raised by 5.02 million USD.Besides, the performance of the new design was investigated under various boiler loads. In summary,the proposal is extremely feasible from the viewpoints of thermodynamics and economics. The currentresearch has been one of the first attempts to integrate the energy conversions of municipal solid wasteand biomass, which lays the groundwork for future research.
Author Contributions: Conceptualization, P.P. and H.C.; data curation, M.Z., H.C. and X.S.; formal analysis,P.P. and M.Z.; funding acquisition, G.X. and H.C.; investigation, P.P., M.Z. and X.S.; software, M.Z.; supervision,G.X. and T.L.; writing—original draft, P.P. and M.Z.; and writing—review and editing, G.X. and H.C. All authorshave read and agreed to the published version of the manuscript.
Funding: This work was supported by the National Nature Science Fund of China (No. 51806062) and theFundamental Research Funds for the Central Universities (No. 2020MS006).
Conflicts of Interest: The authors declare no conflict of interest.
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